President Washington 1st edition receiver sensitivity help

[Lkaskel]

Hi Gang, I have a near mint President Washington (858) that I have recapped and aligned. The radio is performing perfectly except for the receiver sensitivity. On SSB it drops out at -100db and should be ok until around -125 to -130. I have other radios to compare it to and to also assure that my test gear is working ok. Also, the RF gain range (VR2) alignment pot does not seem to have any effect on it.

Any thoughts? The receive works. What could cause it to not be as sensitive?

 

[loosecannon]

have you removed the brown goop yet?

that chassis is so old, it might be a bit conductive and effectively adding a meg ohm resistor between two points that shouldn't be connected, thereby causing reduced sensitivity.

get some GC electronics radio and TV solvent, and use it to soften up and remove the glue.
this is just a first blush guess.
LC

 

[nomadradio]

I don't remember seeing the corrosive contact cement in that radio.

First diagnostic step for a weak receiver is to check the alignment.

The RatShack TRC-449 is a mobile radio with the same circuit board as the 1978 Washington. Full service manual and schemo are here http://www.cbtricks.com/radios/realistic/trc_449/index.htm

A common fault is a failed resonating capacitor hidden inside one or more of the slug-tuned coils. Any of them that appears to show a peak with the slug DEAD EVEN with the rim of the hole has a failed cap inside it. A working part will always show a peak with the slug at least one and a half or more turns below the rim of the hole.

See if any of them appear to show a peak with the slug in that exact position.

If not, the problem is elsewhere.

And if so, let us know which coil shows this symptom. There is usually a cheaper and easier fix for this than removing the old coil and tracking down a new replacement.

73

 

[loosecannon]

Nomad,
just looked at my NOS 858 washington, and i guess you are right.
there is some 'glue' in there, but it doesnt look like the stuff im used to seeing in the 8719 chassis.

it looks like brown rubber cement, which i believe is a 3M product the manufacturers used to use to seat LEDs in faceplates and the like,
my washington has this stuff in it.

i haven't gotten to the restoration of this radio yet, so i don't know whether or not its gotten conductive.

does this sound like what you've found inside these 858s?
LC

 

[nomadradio]

I have seen the corrosive glue in a lot of stuff made since about 1980 or so, but never in the 1978 radios.

Pretty sure the uPD858 models were all discontinued by 1979.

73

 

[Lkaskel]

I don't remember seeing the corrosive contact cement in that radio.

First diagnostic step for a weak receiver is to check the alignment.

The RatShack TRC-449 is a mobile radio with the same circuit board as the 1978 Washington. Full service manual and schemo are here http://www.cbtricks.com/radios/realistic/trc_449/index.htm

A common fault is a failed resonating capacitor hidden inside one or more of the slug-tuned coils. Any of them that appears to show a peak with the slug DEAD EVEN with the rim of the hole has a failed cap inside it. A working part will always show a peak with the slug at least one and a half or more turns below the rim of the hole.

See if any of them appear to show a peak with the slug in that exact position.

If not, the problem is elsewhere.

And if so, let us know which coil shows this symptom. There is usually a cheaper and easier fix for this than removing the old coil and tracking down a new replacement.

73

OK, so the coil #8 did have an interesting issue at one point but it is ok now. That issue was that the coil would peak with the slug removed. When you began inserting the slug the peak reading would drop. I actually walked away for about a day and went through the entire alignment again and the coil worked correctly. I was at a loss. All of the other slugs are a few turns down into the coils. It's really strange. The receive goes from what I would consider "normal", S-6 at -90 DB and then S-1 or less at -100 DB. Its such a drastic drop in comparison to a normal radio. Not that I use the S meter as an accurate measurement but just as a reference to what can be heard. Is there a way to test a coil? Also, what would be an easier fix that removing the coil? I do have many parts radios (no 858 boards) so I guess I could try to find a similar one.

Thanks as always!!!!

 

[Lkaskel]

Nomad,
just looked at my NOS 858 washington, and i guess you are right.
there is some 'glue' in there, but it doesnt look like the stuff im used to seeing in the 8719 chassis.

it looks like brown rubber cement, which i believe is a 3M product the manufacturers used to use to seat LEDs in faceplates and the like,
my washington has this stuff in it.

i haven't gotten to the restoration of this radio yet, so i don't know whether or not its gotten conductive.

does this sound like what you've found inside these 858s?
LC

Not much glue in my radio. I do remove it in all of the radios I work on. The main place this one had glue was around the audio caps. That was it. Now in my Teaberry Stalker XV there was some challenging glue to be removed.....